CO2-Selective membrane process and system for reforming a fuel to hydrogen for a fuel cell

ABSTRACT

The present invention provides a CO 2 -selective membrane process that is useful for the purification and/or water gas shift reaction of a reformed gas, generated from on-board reforming of a fuel, e.g., hydrocarbon, gasoline, diesel, methanol or natural gas, to hydrogen for fuel cell vehicles. Another embodiment of the present invention is directed toward a composition comprising a hydrophylic polymer and at least one ammonium halide salt, the ammonium halide salt being present in an amount ranging from about 10 to 80 wt % based on the total weight of the composition. The composition is suitable in formation of a membrane useful for separating CO 2  from a CO 2 -containing gas, particularly from an on-board reformed gas for the CO 2 -selective membrane process.

This application is a continuation under 37 CFR 1.53(b) of U.S. Ser. No. 08/904,718, filed Aug. 1, 1997 now abandoned.

FIELD OF THE INVENTION

The present invention relates to a membrane process and system for the purification of a fuel, e.g., hydrocarbon, gasoline, diesel, methanol, ethanol or natural gas, to hydrogen for fuel cells. The purification process selectively reomves CO₂ from the reformed product thereby enriching the reformed product in H₂ and increasing the H₂/CO₂ ratio. This invention also relates to a polymer composition suitable for forming a membrane that is useful for separating CO₂ from a CO₂-containing gas stream in the purification process. The present invention is particularly useful when the process is carried-out on-board a vehicle using a fuel cell for transportation.

BACKGROUND OF THE INVENTION

Reforming of a fuel, e.g., hydrocarbon, gasoline, diesel, methanol, ethanol or natural gas, to hydrogen is generally proceeded with the formation of the synthesis gas of CO and H₂ first. For example, steam reforming of methanol with a NiO/Al₂O₃ catalyst at 300-400° C. (T. B. Su and M. H. Rei, J. Chin. Chem. Soc. (Taipei), 38, 535 (1991)) gives the synthesis gas:

CH₃OH→CO+2H₂  (1)

Steam reforming of CH₄ with a nickel-based catalyst at 800° C. is:

CH₄+H₂O→CO+3H₂  (2)

Partial oxidation of CH₄ is:

 CH₄+½O₂→CO+2H₂  (3)

Similarly, partial oxidation of other hydrocarbons, e.g., gasoline and diesel, produces the synthesis gas:

C_(n)H_(2n+2)+{fraction (n/2)}O₂→n CO+(n+1)H₂  (4)

where n is an integer. In the partial oxidation, the synthesis gas produced does not contain N₂ when O₂ is used. If air is used instead of O₂, the synthesis gas produced contains N₂.

The synthesis gas is then sent conventionally to two-stage water gas shifters, in which CO is converted to CO₂ via the water gas shift reaction:

CO+H₂O→CO₂+H₂  (5)

Typically, the first-stage shifter operates at higher temperature than the second- stage shift, e.g., 373° C. for the first stage and 225° C. for the second stage. For the water gas shift reaction, CuO/ZnO/Al₂O₃ catalysts can be used. The product gas from steam reforming of methanol under the optimum conditions at 1 atm and 227° C. with a water rich feed (water/methanol =1.5) contains approximately 66% H₂, 21% CO₂, 1% CO, and 12%,H₂O (J. C. Amphlett, M. J. Evans, R. A. Jones, R. F. Mann, and R. D. Weir, Can. J. Chem. Eng., 59, 720 (1981)).

In some reforming cases, such as the steam reforming of methanol with a CuO/ZnO/Al₂O₃ catalyst, methanol is converted directly and predominantly to CO₂ and H₂:

This reaction operates at temperatures lower than 260° C. with methanol conversion as high as 90%; however, trace CO appears at temperatures above 300° C. and high methanol conversions of about 90% (C. J. Jiang, D. L. Trimm, and M. S. Wainwright, Appl. Catal. A, 93, 245 (1993)).

Japanese Patents 04,321,502 and 04,325,402 claim processes employing H₂-selective membranes, which selectively pass H₂ and reject other gases, for hydrogen manufacture for fuel cells. However, these processes suffer from a low pressure for the H₂ product gas which is much lower than the pressure for the feed gas. Thus, a compressor is needed to compress the product gas to the pressure of the feed gas. In addition, these processes also usually have other shortcomings, such as low H₂ recovery, large membrane areas, and a high CO₂ concentration in the product gas.

It is an object of the present invention to provide a CO₂-selective membrane process that selectively passes CO₂ over H₂ and other gases and that is useful for the purification and/or water gas shift reaction of a reformed gas, generated from reforming of a fuel, e.g., hydrocarbon, gasoline, diesel, methanol, ethanol or natural gas, to hydrogen for fuel cells. This CO₂-selective membrane process can be more advantageous than H₂-selective membrane processes in terms of H₂ product pressure (to avoid the need of a compressor for the product gas), H₂ recovery, membrane area, and CO₂ concentration. Another object of the present invention is to provide a novel polymer composition that is suitable in formation of a membrane useful for the CO₂-selective membrane process. Membranes disclosed in U.S. Pat. No. 5,611,843 may also be used for the CO₂ selective membrane process.

SUMMARY OF THE INVENTION

The present invention is a process and system to purify a fuel feedstream so that the feedstream is enriched in H₂. In general, the process includes the steps of reforming the feedstream, and separating CO₂ with a membrane that selectively removes CO₂ from the feedstream. For most fuel feedstreams, a step of water gas shift reaction is also included in the process. The CO₂ selectively permeable membrane may also be used to perform both steps of enhancing water gas shift reaction and separating CO₂. The process further comprises the step of methanating the H₂-enriched feedstream.

If the fuel feedstream is methanol, then a water gas shift reaction step is not necessary. In this embodiment, the CO₂ selectively permeable membrane may be used to perform both steps of reforming and separating.

In a preferred embodiment, the process is carried-out on board a vehicle that uses a fuel cell for transportation.

Another embodiment of the present invention is directed toward a composition comprising a hydrophilic polymer and at least one ammonium halide salt, the ammonium halide salt being present in an amount ranging from about 10 to about 80 wt % based on the total weight of the composition. The composition is suitable in formation of a membrane useful for separating CO₂ from a CO₂-containing gas, particularly from an on-board reformed gas for the CO₂-selective membrane process.

The embodiments of the present invention will become apparent upon a reading of the brief description of the drawings and the detailed description of the invention which follow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the flow diagram for the first case of the CO₂-selective membrane process of the present invention, in which a CO₂-selective membrane is incorporated into the water gas shifter to become the water gas shifter and CO₂-selective membrane reactor for enhancing the conversion of the water gas shift reaction and for purifying the H₂ product via CO₂ removal.

FIG. 2 illustrates the flow diagram for the second case of the CO₂-selective membrane process of the present invention, in which a CO₂-selective membrane separator is used to purify the H₂ product from the water gas shift converter.

FIG. 3 exhibits the flow diagram for the third case of the CO₂-selective membrane process of the present invention, in which a CO₂-selective membrane is incorporated into the steam reformer for methanol to become the reformer and CO₂-selective membrane reactor for increasing the conversion of the methanol steam reforming reaction and for purifying the H₂ product via CO₂ removal.

FIG. 4 shows the flow diagram for the fourth case of the CO₂-selective membrane process of the present invention, in which a CO₂-selective membrane separator is used to purify the H₂ product from the steam reforming of methanol.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a CO₂-selective membrane process that is useful for the purification of a fuel, e.g., hydrocarbon, gasoline, diesel, methanol, ethanol or natural gas, to hydrogen for fuel cells. This process includes reforming the fuel feedstream and separating CO₂ from the feedstream. The CO₂ is removed by contacting a CO₂-containing gas stream with one side of a nonporous, CO₂ selectively permeable membrane whereby CO₂ is selectively transported over other gases through the membrane and withdrawing from the other side of the membrane a permeate containing CO₂ whereby CO₂ is selectively removed from the gaseous stream. The permeate comprises the CO₂ in increased concentration relative to the feed stream. By “permeate” is meant that portion of the feed stream which is withdrawn at the second side of the membrane, exclusive of other fluids such as sweep gas or liquid which may be present at the second side of the membrane. Air or nitrogen may be used as the sweep gas.

There are four embodiments of the process and the system for performing the process.

In the first case of the present invention, a CO₂-selective membrane is incorporated into the water gas shifter to become the water gas shifter and CO₂-selective membrane reactor for enhancing the conversion of the water gas shift reaction (Eq.(5)) and for purifying the H₂ product via CO₂ removal. FIG. 1 shows the flow diagram for the CO₂-selective membrane process of the present invention, consisting of a reformer, a water gas shifter and CO₂-selective membrane reactor, a methanator, and a fuel cell. As shown in this figure, the H₂ product with low CO₂ and CO concentrations is sent to the methanator, in which CO₂ and CO are converted to CH₄ via the methanator reactions:

CO₂+4H₂→CH₄+2H₂O  (7)

CO+3H₂→CH₄+H₂O  (8)

For enhancing the methanation reactions (Eqs. (7) and (8)), a catalyst of Ru supported on TiO₂ may be used. Finally, the high-purity H₂ product with CH₄ is fed to the fuel cell to drive the electric vehicle. The H₂ product without CO and CO₂ is the most desirable fuel for the fuel cell since CO is a poison for the fuel cell and CO₂ produces CO via the reverse reaction of Eq.(5). To speed up the removal of CO₂ from the permeate side of the CO₂-selective membrane process, a sweep gas, such as air, may be used. The exit stream of the sweep gas may be combined with the anode exhaust stream of the fuel cell to be burned for supplying heat, for example, to the steam reformer or for other heat integration.

The second case of the present invention involves the use of a CO₂-selective membrane separator to purify the H₂ product from the water gas shifter converter. FIG. 2 gives the flow diagram for the CO₂-selective membrane process, consisting of a reformer, a water gas shifter, a CO₂-selective membrane separator, a methanator, and a fuel cell.

As mentioned earlier, in some reforming cases, such as the steam reforming of methanol with a CuO/ZnO/Al₂O₃ catalyst, methanol is converted directly and predominantly to CO₂ and H₂The third case of the present invention is to incorporate a CO₂-selective membrane into the methanol steam reformer to become the reformer and CO₂-selective membrane reactor for increasing the conversion of the steam reforming reaction (Eq.(6)) and for purifying the H₂ product via CO₂ removal. FIG. 3 shows the flow diagram for the CO₂-selective membrane process of this invention, consisting of a reformer and CO₂-selective membrane reactor, a methanator, and a fuel cell.

The fourth case of the present invention uses a CO₂-selective membrane separator to purify the H₂ product from the steam reforming of methanol shown in Eq.(6). FIG. 4 illustrates the flow diagram for the CO₂-selective membrane process, consisting of a reformer, a CO₂-selective membrane separator, a methanator, and a fuel cell.

Another embodiment of the present invention is directed toward a composition comprising a hydrophilic polymer and at least an ammonium halide salt, the ammonium halide salt being present in an amount ranging from about 10 to about 80 wt % based on the total weight of the composition and preferably about 40 to about 65 wt %.

The hydrophilic polymers suitable in the practice of the present invention include polyvinylalcohol, polyvinylpyrrolidone, polyethyleneoxide, polypropyleneoxide, polyacrylamide, polyvinylacetate, blends and copolymers thereof. In general, these polymers will have weight average molecular weights in the range of about 30,000 to 2,000,000 and preferably in the range from about 50,000 to 200,000. Particularly preferred polymers useful in the present invention are polyvinylalcohols having molecular weights in the range from about 50,000 to 150,000.

The ammonium halide salts in the compositions of the present invention are selected from salts having the formulae and mixtures thereof:

(R)₄N⁺X⁻

wherein R is hydrogen or an alkyl group having from 1 to 4 carbon atoms and X is a halide, the halide being selected from the group consisting of fluoride, chloride, bromide, iodide, and mixtures thereof.

As previously stated, the amount of ammonium halide salt to be present in the composition is in the range from about 10 to 80 wt % based on the total weight of the composition, and preferably about 40 to about 65 wt %.

The compositions of the present invention are prepared by first forming a solution of the polymer and the ammonium halide salt in a suitable solvent such as water. Generally, the amount of water employed will be in the range from about 70% to 95%. The composition can then be recovered from the solution by removing the solvent, for example, by allowing the solvent to evaporate; however, it is preferred to use the solution in forming a nonporous membrane. Thus, the resulting solution is formed into a nonporous membrane by techniques well known in the art. For example, the polymer solution can be cast onto a solid support with techniques such as “knife casting” or “dip casting”. Knife casting, of course, is a process in which a knife is used to draw a polymer solution across a flat surface to form a thin film of the polymer solution of uniform thickness after which the solvent of the polymer solution is evaporated, at ambient or temperatures up to about 200° C., to yield the fabricated membrane. When, for example, a glass plate is used as the flat surface, the membrane can then be removed from the support providing a free standing polymer membrane. When, alternatively, the flat surface used is a non-selective porous support such as porous polytetrafluoroethylene, the resulting membrane is a composite membrane comprising the selective membrane polymer and the support. Dip casting is the process in which the polymer solution is contacted with a non-selective porous support. Then excess solution is permitted to drain from the support, and the solvent of the polymer solution is evaporated at ambient or elevated temperatures as above. The membrane comprises both the polymer and the porous support.

The membranes of the present invention also may be shaped in the form of hollow fibers, tubes, films, sheets and the like.

In an alternate embodiment of the present invention, a cross-linking agent is added to the polymer and ammonium halide salt solution before forming a membrane from it.

Suitable cross-linking agents include formaldehyde, divinyl sulfone, toluene diisocyanate, glyoxyal, glutaraldehyde, trimethylol melamine, terephthalatealdehyde, epichlorohydrin, vinyl acrylate, and maleic anhyride. Formaldehyde, divinyl sulfone and toluene dissocyanate are particularly preferred.

The amount of cross-linking agent employed will be in the range of about 1 to about 20 wt % based on the total weight of the solid composition formed from the solution.

Membranes formed from the solution containing a cross-linking agent typically are heated at a temperature and for a time sufficient for cross-linking to occur. Generally, cross-linking temperatures in the range from about 80° C. to about 200° C. are employed. Cross-linking will occur in from about 1 to 72 hours.

As indicated previously, the compositions of the present invention are suitable for use as a nonporous membrane for separating CO₂ from a CO₂-containing gas stream, particularly from an on-board reformed gas for the CO₂-selective membrane process of the present invention.

The present invention will be better understood by reference to the following examples which are offered by way of illustration not limitation.

EXAMPLES

In the examples which follow, the separation factor (selectivity) for CO₂ vs. H₂ is expressed as follows: $\begin{matrix} {{SeparationFactor} = \frac{{{CO}_{2}/H_{2}}\quad \text{concentration~~ratio~~in~~the~~permeate}}{{{CO}_{2}/H_{2}}\quad \text{concentration ratio in the retentate}}} & (9) \end{matrix}$

The retentate refers to the mixture on the feed side of the membrane which is rejected by the membrane under the operating conditions. Permeability is expressed in Barrer (1 Barrer=10⁻¹⁰ cm³(STP)·cm/(cm²·s·cm Hg)). The permeability is determined by the use of the relationship between the permeability P and the permeation rate Q (cm³(STP)/s) as follows:

Q=A P (p₁−p₂)/L  (10)

where A is the membrane area, P₁ and P₂ are the CO₂ partial pressures in the retentate and permeate streams, respectively, and L is the membrane thickness. P/L is called the permeance with a typical unit of GPU (gas permeation unit, 1 GPU=10⁻⁶cm³(STP)/(cm^(2·s·)cm Hg)). The partial pressures are determined based on concentration measurements by gas chromatography and total pressure measurements by pressure gauges. The permeation rate is determined based on concentration measurements obtained by gas chromatography and permeate stream flow rate measurements by a flow meter.

Example 1 CO2-Selective Membrane Process for Purification of a Reformed Gas Containing N₂ at 3 atm

The CO₂-selective membrane process of the present invention is used to purify a reformed gas to supply H₂ to a 10 KW fuel cell with a flow diagram shown in FIG. 2 or 4. It should be noted that a fuel cell vehicle may require several 10 KW fuel cells, e.g., 5 (50 KW total). The 10 KW fuel cell requires a hydrogen feed rate of 0.07 mole H₂/s. The reformed gas contains 40% H₂, 19% CO₂, 1% CO, and 40% N₂ (on a water-free basis) at a total pressure of 3 atm. The CO₂-selective membrane used has a CO₂ permeance of 600 GPU, a CO₂/H₂ selectivity of 75, a CO₂/CO selectivity of 100, and a CO₂/N₂ selectivity of 100. Air is used as the sweep gas for the permeate to have H₂, CO₂, CO and N₂ partial pressures in the permeate to be insignificant in comparison with their partial pressures in the retentate. By the use of Eq. (10), the membrane area, H₂, CO and N₂ concentrations in the retentate, and H₂ recovery are calculated for a CO₂ concentration of 0.5% in the retentate. The membrane area is 92.5 ft², the retentate, i.e., the product gas, has 49.0% H₂, 0.5% CO₂, 1.2% CO, and 49.3% N₂ at a total pressure of 3 atm, and the H₂ recovery is 97.2% of the H₂ available in the reformed gas. The retentate is then treated in a methanator to convert CO₂ and CO to CH₄, resulting in a desirable fuel without containing CO₂ and CO for the fuel cell.

Example 2 CO₂-Selective Membrane Process for Purification of a Reformed Gas Containing N₂ at 10 atm

The CO₂-Selective Membrane Process of the present invention for purification of a reformed gas containing 40% H₂, 19% CO₂, 1% CO, and 40% N₂ (on a water-free basis) at a total pressure of 10 atm is the same as that described in Example 1 except the total pressure. The higher the total pressure (at the same gas composition), the smaller the membrane area. Again, Eq. (10) is used to calculate the membrane area, H₂, CO and N₂ concentrations in the retentate, and H₂ recovery for a CO₂ concentration of 0.5% in the retentate. The membrane area is 27 ft², the retentate (product gas) has 49.0% H₂, 0.5% CO₂, 1.2% CO, and 49.3% N₂ at a total pressure of 10 atm, and the H₂ recovery is 97.2% of the H₂ available in the reformed gas. Similarly, the retentate is then treated in a methanator to convert CO₂ and CO to CH₄, yielding a desirable fuel cell fuel without containing CO₂ and CO.

Example 3 H₂-Selective Membrane Process for Purification of a Reformed Gas Containing N₂ at 10 atm (for Comparison)

For comparison, the membrane area and H₂, CO₂, CO and N₂ concentrations in the permeate, i.e., the product gas, for an H₂-selective membrane process with 80% H₂ recovery from the same reformed gas described in Example 2 are calculated in the same way as in Example 2 via Eq. (10) for the 10 KW fuel cell. The membrane process uses a state-of-the-art, commercially available H₂-selective membrane (polyimide) with a H₂ permeance of 100 GPU, a H₂/CO₂ selectivity of 10, a H₂/CO selectivity of 100, and a H₂/N2 selectivity of 170 (W. S. Winston Ho and Kamalesh K. Sirkar, Membrane Handbook, p. 44, Chapman & Hall, New York (1992)), and this process gives the product gas (permeate) at 1 atm (without the use of a sweep gas). The membrane area is 126 ft², and the product gas (permeate) contains 88.3% of H₂, 10.1% CO₂, 0.06% CO, and 1.5% N₂. Table 1 shows the comparison of the CO₂-selective membrane process of the present invention (Example 2) with the H₂-selective membrane process (Example 3). As shown in this table, the membrane process of this invention gives much higher product gas pressure (10 atm vs. 1 atm), greater H₂ recovery (97.2% vs. 80%), and less membrane area required (27 ft² vs. 126 ft²) than the H₂-selective membrane process. In addition, the former process gives a lower combined concentration of carbon oxides in the product gas (0.5% CO₂ and 1.2% CO vs. 10.1% CO₂ and 0.06% CO) than the latter process. The product gas of the former process is suitable for methanation to yield a desirable fuel cell fuel without carbon oxides, whereas the product gas of the latter process has too much CO₂ to be practically suitable for methanation. Therefore, the process of this invention is more effective and advantageous than the H₂-selective membrane process.

TABLE 1 Comparison of the CO₂-Selective Membrane Process with a H₂-Selective Membrane Process for Purification of a Reformed Gas Containing N₂* Product Gas Composition Total Product H₂ Membrane (mole %) Gas Pressure Recovery Area Process H₂ CO₂ CO N₂ (atm) (%) (ft²) CO₂-Selective 49.0 0.5 1.2 49.3 10 97.2 27 Membrane Process, This Invention (Example 1) H₂-Selective 88.3 10.1 0.06 1.5 1 80 126 Membrane Process (Example 3) *The reformed gas contains 40% H₂, 19% CO₂, 1% CO, and 40% N₂ at total pressure of 10 atm.

Example 4 CO₂-Selective Membrane Process for Purification of a Reformed Gas without Containing N₂ at 10 atm

Similar to the process described in Example 1, the CO₂-selective membrane process of the present invention is employed to purify a reformed gas to supply H₂ to a 10 KW fuel cell with a flow diagram shown in FIG. 2 or 4. As mentioned earlier, a fuel cell vehicle may require several 10 KW fuel cell, e.g., 5 (50 KW total). The 10 KW fuel cell requires a hydrogen feed rate of 0.07 mole H₂/s. The reformed gas does not contain N₂, and it has a composition of 74.9% H₂, 24% CO₂, and 1.1% CO (on a water-free basis) at a total pressure of 10 atm. The CO₂-selective membrane used, which has a composition of 50 wt % tetramethylammonium fluoride salt and 50 wt % polyvinylalcohol described in Examples 6 and 7, has a CO₂ permeance of 600 GPU and a CO₂/H₂ selectivity of 19. Air is used as the sweep gas for the permeate to have H₂, CO₂ and CO partial pressures in the permeate to be insignificant in comparison with their partial pressures in the retentate. Again, Eq. (10) is used to calculate the membrane area, H₂ and CO concentrations in the retentate, and H₂ recovery for a CO₂ concentration of 1.2% in the retentate and a CO₂/CO selectivity of 100. The membrane area is 17 ft², the retentate (product gas) has 97.1% H₂, 1.2% CO₂, and 1.7% CO at a total pressure of 10 atm, and the H₂ recovery is 83% of the H₂ available in the reformed gas. Again, the retentate is then treated in a methanator to convert CO₂ and CO to CH₄, giving a desirable fuel without containing CO₂ and CO for the fuel cell.

Example 5 H₂-Selective Membrane Process for Purification of a Reformed Gas without Containing N₂ at 10 atm (for Comparison)

For comparison, the membrane area and H₂, CO₂ and CO concentrations in the permeate, i.e., the product gas, for a H₂-selective membrane process with 80% H₂ recovery from the same reformed gas described in Example 4 (without containing N₂) are calculated in the same way as in Example 4 via Eq. (10) for the 10 KW fuel cell. The membrane process employs the state-of-the-art H₂-selective membrane (polyimide) described in Example 3 with a H₂ permeance of 100 GPU, a H₂/CO₂ selectivity of 10, and a H₂/CO selectivity of 100, and this process yields the product gas (permeate) at 1 atm (without the use of a sweep gas). The membrane area is 45 ft², and the product gas (permeate) contains 92.7% H₂, 7.3% CO₂, and 0.04% CO. Table 2 shows the comparison of the CO₂-selective membrane process of the present invention (Example 4) with the H₂-selective membrane process (Example 5). As shown in this table, the membrane process of this invention gives higher H₂ concentration (97.1% vs. 92.7%), i.e., lower combined concentration of carbon oxides (1.2% CO₂ and 1.7% CO vs. 7.3% CO₂ and 0.04% CO), much higher product gas pressure (10 atm vs. 1 atm), greater H₂ recovery (83% vs. 80%), and less membrane area required (17 ft² vs. 45 ft²) than the H₂-selective membrane process. In addition, the product gas of the former process with a lower combined concentration of carbon oxides is suitable for methanation to yield a desirable fuel cell fuel without carbon oxides, whereas the product gas of the latter process has too much CO₂ to be practically suitable for methanation. Thus, the process of this invention is again more effective and advantageous than the H₂-selective membrane process.

TABLE 2 Comparison of the CO₂-Selective Membrane Process with a H₂-Selective Membrane Process for Purification of a Reformed Gas without Containing N₂* Product Gas Composition Total Product H₂ Membrane (mole %) Gas Pressure Recovery Area Process H₂ CO₂ CO (atm) (%) (ft²) CO₂-Selective 97.1 1.2 1.7 10 83 17 Membrane Process, This Invention (Example 1) H₂-Selective 92.7 7.3 0.04 1 80 45 Membrane Process (Example 3) *The reformed gas contains 74.9% H₂, 24% CO₂, 1.1% CO, at total pressure of 10 atm.

Example 6 Synthesis of 50 wt % Tetramethylammonium Fluoride Salt and 50 wt % Polyvinylalcohol Membrane

To 18.65 g of water was added 3.0 g of polyvinylalcohol (PVA) with stirring and heating at about 75° C. until a clear solution of the polymer was obtained. Separately, 5.32 g of a tetramethylammonium fluoride salt with 4 H₂O ((CH₃)₄NF.4 H₂O), which contained 3.0 g of tetramethylammonium fluoride salt, was dissolved in 5 g of water. This solution was added to the PVA solution with stirring for about 15 minutes to obtain a clear, homogeneous solution. The resulting solution was then centrifuged while cooling for about 30 minutes. Following centrifugation, a membrane was knife-cast with a gap setting of 8 mils onto a support of microporous polytetrafluoroethylene. Water was allowed to evaporate from the membrane overnight in a nitrogen box at ambient conditions. The membrane was then heated in an oven at 90° C. for 5 hours. The resulting membrane comprised about 60 wt % tetramethylammonium fluoride salt and 50 wt % polyvinylaclohol on the microporous polytetrafluoroethylene support, and had a thickness of 49 microns (exclusive of the support).

Example 7 Permeation Measurement of Membrane of Example 6

In the permeation measurement to evaluate the separation factor (selectivity) of CO₂ vs. H₂ and the permeability of CO₂, the membrane was placed in a permeation cell comprising the first compartment for contacting a feed stream against the upstream side of the membrane and the second compartment for withdrawing the permeate from the downstream side of the membrane. The active membrane area in the cell was 63.62 cm². A feed gas comprising 75% H₂ and 25% CO₂ under a total pressure of about 3 atm at ambient temperature (23° C.) was contacted against the membrane at a flow rate of 120 cm³/min. The permeate was swept by nitrogen under a pressure of about 1 atm and a total flow rate of 50.9 cm³/min for the permeate/nitrogen stream. Both the feed and the sweep streams were humidified by bubbling through deionized water prior to contacting the membrane.

For the membrane of Example 6 comprising 50 wt % tetramethylammonium fluoride salt and 50 wt % polyvinylalcohol, the CO₂/H₂ selectivity result obtained was 19, and the C₂ permeability was 348 Barrers.

As shown in Example 7, the membrane of this invention may be employed for removal of CO₂ from a CO₂-containing gas, e.g., a reformed gas. The use of the membrane composition in the CO₂-selective membrane process of the present invention for the purification of a reformed gas for a fuel cell vehicle has been described in Example 4. 

What is claimed is:
 1. A system for converting a fuel feedstream to a hydrogen rich and CO₂ and CO deficient feedstream comprising a reformer, a water gas shifter, comprising CO₂-selective membrane, and a methanator, wherein said membrane is a composition comprising a hydrophilic polymer and at least one ammonium halide salt, the ammonium halide salt being present in an amount ranging from about 10 to about 80 wt % based on the total weight of the composition, wherein the ammonium halide salt is selected from ammonium halide salts having the formulae and mixtures thereof: (R)₄N⁺X⁻ wherein R is hydrogen or an alkyl group having from 1 to 4 carbon atoms and X is a halide, the halide being selected from the group consisting of fluoride, chloride, bromide, iodide, and mixtures thereof.
 2. The system of claim 1 further comprising a fuel cell.
 3. The system of claim 2 wherein said system is on-board a vehicle that is driven by said fuel cell.
 4. The system of claim 1 wherein said hydrophilic polymer is selected from the group consisting of polyvinylalcohol, polyvinylpyrrolidone, polyethyleneoxide, polypropyleneoxide, polyacrylamide, polyvinylacetate, blends thereof and copoloymers thereof.
 5. The system of claim 1 wherein said polymer is polyvinylalcohol.
 6. The system of claim 1 wherein said composition includes from about 1 to about 20 wt % of a cross-linking agent based on the total weight of the composition.
 7. The system of claim 1 wherein said cross-linking agent is selected from the group consisting of formaldehyde, divinyl sulfone, toluene disocyanate, glyoxal, glutaraldehyde, trimethylol melamine, terephthaladehyde, epichlorohydrin, vinyl acrylate, and maleic anhydride.
 8. The system of claim 1 wherein the ammonium halide salt is tetramethylammonium fluoride salt.
 9. A system for converting a fuel feedstream to a hydrogen rich and CO₂ and CO deficient feedstream comprising a reformer, a water gas shifter, CO₂ separator comprising CO₂-selective membrane, and a methanator, wherein said membrane is a composition comprising a hydrophilic polymer and at least one ammonium halide salt, the ammonium halide salt being present in an amount ranging from about 10 to about 80 wt % based on the total weight of the composition, wherein the ammonium halide salt is selected from ammonium halide salts having the formulae (R)₄N⁺X⁻an mixtures thereof, wherein R is hydrogen or an alkyl group having from 1 to 4 carbon atoms and X is a halide, the halide being selected from the group consisting of fluoride, chloride bromide, iodide, and mixtures thereof.
 10. The system of claim 9 further comprising a fuel cell.
 11. The system of claim 10 wherein said system is on-board a vehicle that is driven by said fuel cell.
 12. A system for converting a fuel feedstream to a hydrogen rich and CO₂ and CO deficient feedstream comprising a reformer comprising CO₂-selective membrane, and a methanator, wherein said membrane is a composition comprising a hydrophilic polymer and at least one ammonium halide salt, the ammonium halide salt being present in an amount ranging from about 10 to 80 wt % based on the total weight of the composition, wherein the ammonium halide salt is selected from ammonium halide salts having the formulae (R)₄N⁺X⁻an mixtures thereof, wherein R is hydrogen or an alkyl group having from 1 to 4 carbon atoms and X is a halide, the halide being selected from the group consisting of fluoride, chloride, bromide, iodide, and mixtures thereof.
 13. The system of claim 12 further comprising a fuel cell.
 14. The system of claim 13 wherein said system is on-board a vehicle that is driven by said fuel cell.
 15. A system for converting a carbon fuel feedstream to a hydrogen rich and CO₂ and CO deficient feedstream comprising of a reformer, CO₂ separator comprising CO₂-selective membrane, and a methanator, wherein said membrane is a composition comprising a hydrophilic polymer and at least one ammonium halide salt, the ammonium halide salt being present in an amount ranging from about 10 to about 80 wt % based on the total weight of the composition, wherein the ammonium halide salt is selected from ammonium halide salts having the formulae (R)₄N⁺X⁻an mixtures thereof, wherein R is hydrogen or an alkyl group having from 1 to 4 carbon atoms and X is a halide, the being selected from the group consisting of fluoride, chloride, bromide, iodide, and mixtures thereof.
 16. The system of claim 15 further comprising a fuel cell.
 17. The system of claim 16 wherein said system is on-board a vehicle that is driven by said fuel cell. 